In a motor vehicle, this on-board network is used to supply the various items of electrical equipment with which the vehicle is equipped. The supply is provided by at least one battery. The battery is recharged by means of a rotary electrical machine, by energy supplied by the rotation of the thermal engine of the vehicle. Rotary electrical machine means more generally any polyphase rotary electrical machine which is used for the production of direct output current which supplies the on-board network. In particular it can be an alternator or an alternator-starter.
In the event of sudden disconnection of an electric load of the on-board network, or of a battery, or both, a load dump phenomenon is created which gives rise to excess voltage on the on-board network. In fact, since regulation of a current inductor in the machine cannot act rapidly enough after the load dump, the machine continues to supply the same output current, whereas the current consumption of the on-board network side has dropped.
Conventionally, the vehicle battery is a 14 V battery. In principle, because of its low internal resistance, it limits to approximately 17 V the voltage peaks which occur on the vehicle on-board network in the event of load dump. This battery thus absorbs the small excess voltages. Nevertheless, in the event of disconnection of the battery (as a result of breakage of a supply cable, for example), a very high excess voltage can occur on the on-board network. In fact, since the inductor current of the machine cannot be turned off instantaneously, the latter generates an electromotive force and a current which are proportional respectively to the speed of the machine and to the inductor current. This electromotive force then charges the set of condensers which are connected to the on-board network, and consequently increases substantially the direct voltage on the on-board network.
In a known technique, the on-board network voltage is limited to a maximum of 32 V by the rectifier bridge of the machine, with the semiconductors which are not conducting the phase current going alternately into Zener diode mode.
This excess voltage is liable to damage the electrical equipment which is supplied by the on-board network. This is why all the electrical equipment of the vehicle has dimensions such as to withstand a maximum voltage of approximately 32 V, which corresponds to excess voltage of approximately 20 V.
Various solutions are known which make it possible to limit the voltage on the on-board network to an admissible maximum voltage, i.e. the highest voltage which the electrical equipment of the vehicle can withstand without any risk of damage.
The solution described in the article: “A New Design for Automotive Alternators”, by D. J. Perreault et al, published in the proceedings of the Congress “2000 International Congress on Transportation Electronics (Convergence 2000)”, Detroit, Mich., October 2000, pp. 583-594, consists of blocking in the conducting state at least one of the switching transistors of a cut-off rectifier. By this means, at least one of the phase windings is short-circuited. The short-circuit is maintained, and the excitation of the machine is reduced until the voltage has returned to an acceptable level.
This simple method of elimination of the transient phenomena has disadvantages however:                the supply to the equipment connected to the on-board network is not guaranteed because the excitation is not maintained;        the electrical and thermal protection of the semiconductor elements of the rectifier circuit is not ensured.        